I drive a 1985 model car, no MPFI, no electronics and a vibrating-contact voltage-regulator/cut-out system. Additionally, my usage is also limited and practically restricted only to weekends. I wanted a simple device which could monitor the health of my car 12V battery and the charging system.
This Instructable presents a compact device I built based on the Texas Instruments LM3914 LED Dot-Indicator integrated circuit. The circuitry is housed within a defective car mobile charger case and needs only to be plugged into the car cigarette lighter socket.
A unique feature of this device is the provision of a socket which connects to the battery through a fuse permitting trickle charging of the car battery while monitoring the charging voltage.
Check out this Video:
This simplified schematic shows how a typical car +12V bus is formed.
The +12V battery with the -ve connected to body is the primary power source which is augmented by the AC alternator when the engine is running. The alternator output is rectified and feeds the bus, its output is regulated based on a feedback connection. Different loads are switched onto the bus when lamps and other electrical systems are operated.
In operation battery power is used to start the car and then the alternator powers the bus while charging the battery. Under heavy load conditions both the alternator and battery share the power output required and on alternator failure or malfunction the power is again primarily from the battery.
The indicator I have built is connected directly to the bus and is powered by the battery before the engine starts up. While the engine is running it indicates the bus voltage and when there is a malfunction of the alternator it indicates the bus voltage while drawing power for its operation from the battery.